1. Field of the Invention
The present invention relates to a method of fabricating a thin-sheet-coated composite substrate that fabricates a thin-sheet-coated composite substrate by bonding a thin sheet or a film, such as a thin glass sheet, to a substrate formed by laminating optical films or the like with an adhesive and, more particularly, to a method of fabricating a thin-sheet-coated composite substrate having a thin sheet having an improved flatness.
2. Description of the Related Art
Functional substrates coated with a thin glass sheet are employed in electronic parts and optical parts. A conventional method of fabricating a thin-sheet-coated composite substrate superposes a substrate and a thin sheet with an adhesive placed between the substrate and the thin sheet to form a layered structure, holds the layered structure between a pair of highly flat pressure plates, and compresses the layered structure so that the adhesive is spread in an adhesive layer of a predetermined thickness, and then cures the adhesive layer. When the layered structure is compressed between the pressure plates, the adhesive is squeezed out of the space between the substrate and the thin sheet. The squeezed adhesive spreads over portions not to be coated with the adhesive of the substrate, and the thin sheet, and the pressure plates are smeared with the adhesive. Therefore, the pressure plates must be cleaned to remove the adhesive before using the same again. This method is unable to adjust the thickness of the adhesive layer in a satisfactory accuracy and has a difficulty in applying a fixed pressure uniformly to the layered structure in case that the substrate is larger than the thin sheet. Since the layered structure is compressed between the pressure plates, the components of the layered structure are firmly bonded together and it is difficult to separate the bonded components.
A method proposed in JP-A 55-68040 comprises the steps of supporting a substrate by a vacuum chuck on a spinner, dripping a liquid adhesive onto the substrate, putting a thin glass sheet on the adhesive, blowing air against the thin glass sheet to spread the adhesive in an adhesive layer over the entire surface of the substrate, driving the spinner to rotate the substrate so that a surplus portion of the adhesive is scattered and the substrate and the thin glass sheet are bonded firmly together to form a composite substrate, and removing the composite substrate from the vacuum chuck.
Although this previously proposed method eliminated drawbacks in the conventional techniques to a considerable extent, this method has a drawback that the flatness of the thin glass sheet of the composite substrate fabricated by this method is unsatisfactory. The substrate has an irregular thickness and does not have satisfactory flatness unless the substrate is finished by polishing. When a thin glass sheet is attached to the surface of a substrate having an irregular thickness or to a layer formed on such a substrate, the surface of the thin glass sheet reflects the irregular shape of the surface of the substrate underlying the thin glass sheet.